1. Field of the Invention
This invention relates to a process for casting vitreous articles using the lost wax process.
2. Description of the Prior Art
Casting of articles using the lost wax process is one of the oldest metallurgical procedures. First, a pattern of the article to be cast is made using a substance which is vaporized at the temperature employed during casting. Wax is often used for such patterns. The pattern is embedded in a heat resistant silica compound such as sand or microballs of a refractory oxide which surrounds the pattern to form a casting mold. The sand or microballs are contained in a receptacle called a casting shell. During casting, the pattern is vaporized and replaced by the casting material. The gases released are emitted through the casting mold.
Over the years, numerous improvements have been made in the process. Recently, it has been rendered more economical by employing vaporizable patterns of relatively low-cost materials such as polystyrene or polyethylene. However, because of the high fusion temperature and low rate of solidification of vitreous materials, casting of vitreous articles using the lost wax process was regarded as generally impossible. This was primarily due to the fact that the mold could not be filled with the vitreous materials without being damaged.
In casting vitreous materials and particularly glass, a delicate balance exists between the allowable temperatures and viscosities of the vitreous material during the casting operation. For example, if the vitreous material is cast at too low a temperature, it will solidify too rapidly, particularly in the area adjacent to the inner walls of the sand mold, i.e., where the sand mold contacts the surface of the volatizable pattern. Moreover, as the casting operation proceeds, the vitreous material forms a number of loosely cohesive layers in the area adjacent to the inner walls of the mold. When the casting temperature is too low, fragile zones and devitrification zones are formed in this area. These zones can cause the article to break.
Another problem arising when the temperature of the vitreous material during casting is too low is that the gases released as the pattern undergoes volatization are unable to pass through the excessively viscous mass of cast material. This produces imperfections in the finished article.
In casting vitreous articles, the viscosities of the vitreous material during the casting operation is ordinarily maintained at about 10.sup.3 to 10.sup.4 poises. These viscosities correspond to the highest temperatures in the glass working range for casing operations. The glass working range covers viscosities between 10.sup.3 and 10.sup.7 poises. When casting vitreous articles using the lost wax process the viscosities in the glass working range are too high to be successfully employed. Even a viscosity of only 10.sup.3 poises is still too high to enable the utilization of the lost wax process.
It is possible to heat the glass to a temperature at which it flows very well due to its lower viscosity. However, these low viscosities, and their attendant high temperatures, often cause damage to the mold when the two come into contact. To alleviate this problem, the pattern can be coated with a thin layer of a mixture of graphite and a refractory powder before being placed in the casting sand. Yet the vitreous material due to its low viscosity may pierce the layer of graphite and refractory powder and seep into the adjacent layers of sand making it impossible to obtain an article corresponding to the desired pattern. This is especially true when its viscosity is low enough so that the material adequately fills the pattern. By coating the pattern with a thick layer of graphite and refractory powder, this problem can be prevented. The difficulties involved in producing this thick layer, however, would prevent commercial utilization of the process since it would require several successive layers, each requiring additional drying time. Nevertheless, in casting complex articles, for example, a spiral spring or a pattern in the form of a cross with enlarged sections at the ends of its arms, the viscosity of the vitreous material during casting must be low enough so that the vitreous material correctly fills the entire mold. Yet, if the viscosity of the vitreous material is too low the material will pierce the mold.